Joining tubes to tube sheets



Dec. 27, 1960 E. c..cHAPMAN JOINING TUBES To TUBE 'SHEETS Filed March 8,1957 lllll.

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United States Patent O JOINING TUBES T TUBE SHEETS Edward C. Chapman,Chattanooga, Tenn., assignoi to `Combustion Engineering, Inc., New`ttor-k, NX., a corporation of Delaware Filed Mar. 8, 1957, Ser. No.644,805

Claims. (Cl. 257-236) This invention relates to the attachment of tubesto tube sheets, headers or the like in heat exchangers and has specificrelation to a connecting arrangement for overcoming the problem ofstress corrosion cracking experienced with certain materials undercertain conditions.

The term stress corrosion cracking refers to the cracking of metal thatis in a stressed condition and which is contacted by certain chemicalswhich attack the stressed metal in a manner to produce cracking. Theexact conditions necessary for stress corrosion cracking to progresshave never been accurately established but the variables that areinvolved are the concentration of the chemicals in contact with themetal and the amount of stresses present in the metal. One instance ofstress corrosion cracking which is presently causing great concern inthe eld of heat exchangers is the stress corrosion cracking ofaustenitic stainless steel when operating under stresses in watercontaining chlorides, such as NaCl, CaClz and MgCl2. Austeniticstainless steel has been known to crack under stress when the watercontains not more than 18 parts per million of chlorides and whenevaporation of the water is taking place and there are crevices in whichthe chlorides may concentrate (as may prevail with conventional tubeconnecting arrangements) an unlimited degree of concentration may beobtained in these crevices with a very small concentration of chloridesbeing present in the water.

In connecting tubes to tube sheets, headers or the like, the tubes aregenerally expanded into tight engagement with the wall of the opening inthe tube sheet with the extremity of the tube being seal welded to thetube sheet. These operations of course stress the tube metal at thisconnection so that if the conditions are right (i.e., the right metalsand the right chemicals present) stress corrosion cracking of thisstressed metal may take place. One situation where this is the case iswhere austenitic steel tubes are connected with ferritic steel tubesheets in a steam generator organization wherein the water that isevaporated contains chlorides (as most boiler water does) and is incontact with the tubes. Ferritic steel -is not subject to stresscorrosion cracking in the presence of chlorides while, as previouslymentioned, austenitic stainless steel is. In such an instance it isessential that there be no crevices at the tube connection in whichchlorides may concentrate in contact with the austenitic steel tube. Ifconventional tube connections are used it is more than likely that theexpanded joint between the tube and tube sheet will open up to at leasta minor extent since the coefficient of the expansion of the tube andthe tube sheet are different. Once the expanded joint is slightly open asituation prevails where stress corrosion cracking of the tube willundoubtedly occur. As an example of a situation involving the problem,there are presently being built steam generators wherein the heatingmedium is conveyed through tubes and the water that is evaporatedsurrounds the tubes with the design calling for 5/s inch outsidediameter austenitic stainless steel tubes to be connected to a ferriticsteel tube sheet 8 inches thick and with the design being such thataccessibility to the tubes is very diicult rendering it almostimpossible to obtain a good job of expanding the tubes into engagementwith the bore throughout the thickness of the tube sheet.

The present invention eliminates the possibility of stress corrosioncracking of the tube connection in instances such as the one justmentioned and in accordance with the invention, in connecting a tube,which is subject to stress corrosion cracking in the presence of certainchemicals that exist in the environment in which the tube is to be used,to a tube sheet or the like, which is not subject to such cracking inthe presence of these chemicals, a sleeve is positioned over the tubeend that is to be connected to the tube sheet and is brazed to the tubethroughout the overlying surfaces of the sleeve and tube. This sleeve isof a material which is not subject to stress corrosion cracking in thepresence of these chemicals and after being thus secured to the tube,the tube end and sleeve are placed in a suitable bore provided in thetube sheet and by rolling or similar method the tube and sleeve areexpanded so that the sleeve is in tight engagement with the bore. Theextremity of the tube and sleeve are then seal welded to the tubesheet.' With this construction, if the expanded joint between the sleeveand tube sheet should leak stress corrosion cracking, as a result ofconcentration of the chemicals in this joint, will not occur since thematerial of the sleeve and tube sheet are not subjected to such crackingand since the sleeve and tube are integrally brazed'together there willbe no chance for the formation of crevices between these members inwhich the chemicals could possibly con-` centrate and cause cracking ofthe tube.

It is the object of this invention to provide an im.- proved tube andplate or the like connection particularly adaptable for use inevaporators and organized to prevent stress corrosion cracking ofthe-tube at the connection when the Itube is made of a materialsusceptible to such cracking while the tube sheet is not succeptible tosuch cracking.

Other and further objects of the invention will become apparent to thoseskilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises anarrangement, construction and combination i of the elements of theinventive organization in such a manner as to attain the results desiredas hereinafter more particularly set forth in the following detaileddescription of an illustrative embodiment, said embodiment being shownby the acompanying drawing wherein:

Fig. l is a diagrammatic representation in the nature of a verticalsection through a heat exchanger, which takes the form of a steamgenerator, and wherein tubes are interconnected with spaced tube sheetswith the interconnection of the t-ubes with the tube sheets being inaccordance with`the present invention.

Fig. 2 is an enlarged sectional view of the connection of one of thetubes with one of the tube sheets.

Referring now to the drawing, wherein like reference I characters areused throughout to designate like elements, the heat exchanger shown inFig. l comprises a housing or shell 10 within which is provided thespaced tube sheets 12 with these tube sheets being spaced from thestantially lled with waterwhich is supplied through conf duit 24 withthis water of course surrounding the exterior of tubes 18. Sufficientheat is imparted from the heating medium iowing through the tubes sothat a portion of the water in space 26 is converted to steam with thesteam passing upward through connecting'` conduits 23 into. drum 3.0where any residual water is separated fromr the steam and returned tospace 26 through conduit 32 with. the steam leaving the upper portion ofthe drum through conduit 34.

In the illustrative organization the tube sheets I2 are of ferriticsteel while thetubes 18 are of austenit'ic stainlessV steel so thatiteven minor amounts of chlorides are presentv in the water in space 26stress. corrosion cracking of the tubes at the end connections with thetube sheets would prevailif there are crevices present which willpermitA the chlorides to concentrate in engagement with the tubes. Mostboiler water contains suficient chlorides to cause stress corrosioncracking of the austenitic steel tubes under these circumstances withmarine boilers generally having rather large amounts of chlorides in theboiler waten As best shown in Fig. 2, the ends of the tubes 18 areprovided with a sleeve 36 disposed thereover with this sleeve being of amaterial which is not affected by chlorides or in other words, is notsubjected to stress corrosion cracking as a result of chlorides beingpresent in the Water in space 26. Examples of material of which thissleeveV may be made are; ferritic steel and non-ferritic-alloys high inchrome content, although it should be understood that any material maybe employed which will withstand the operating conditions that prevailand which is not subject to stress corrosion cracking as is the tube.Sleeve 36 is brazed to tube 18 so that the overlying surfaces of thesleeve and tube are integrally bonded together throughout their entirearea. Any one of a variety of metals or alloys may bev used as thetiller, indicated as 39, in brazing the sleeve and tube together with analloy consisting of 991/2% silver and 1/2%. lithium having been foundparticularly satisfactory. Since most of the desirable lfillermaterials, such as the one mentioned, are suiciently remote fromaustenitic stainless steel in the electromotive series so as to producetroublesome electrolysis between this ller metal and the tube the tillermetal is sealed from the water in space 26 by means of seal Weld 38which is preferably of a material generally corresponding to tube 18.

After the sleeve 36 is brazed to the end of tube 18 the tube and sleeveare positioned within bore 40 provided in thetube sheet 12. 'Ihe end ofthe tube and the sleeve are then expanded in the conventional manner sothat sleeve 36 is in tight engagement with the wall of bore 40.Thereafter the tube and sleeve are seal welded to the tube sheet bymeans of weld 42 which extends throughout the circumference of the tubeand sleeve.

By connecting tube 18 to tube sheets 12 in this manner the only possibleplace that a crevice may develop is between the sleeve 36 and `tubesheet 12 and since both of: these members are of a material which is notaffected by the presence of chlorides in the Water in space 26 andaccordingly is not subjected to stress corrosion cracking as the resultof building up of concentrations of chlorides in these crevices theproblem of stress corrosion cracking. in such an organization isovercome. Because sleeve 36 and tube 18 are bonded together throughouttheir entireoverlyingl area the juncture of the sleeve and tube is notsubjected to the development of leaks or the formation of crevices. ltis essential that the sleeves and tubes be joined together throughouttheir overlying areas in order to obtain this result since if a portionof the overlying area were not brazed together the unbrazed area wouldhave a tendency to propagate due to the notch effect in the presence ofthermal fatigue stresses. By integrally bonding the sleeve and tubetogether in this manner good heat conductivity between these elements ishad which minimizes thermal stresses.

While- -I haveA illustrated and described` apreterred emd bodiment of myinvention it is to be understood that such is merely illustrative andnot restrictive and that varia` tions and modifications may be madetherein without departing from the spirit and scope of the invention. Itherefore do not wish to be limited to the precise details set forth butdesire to avail myself of such changes as fall within the purview of myinvention.

What I claim is:

l. In an organization of the type described an austenitic steel tube, aferritic steel sleeve disposed on` the end of the tube and brazed to thetube throughout the overlying area of the tube and sleeve with thefiller metal being such as to produce electrolysis between it and thetube in the presence of Water, the end of the sleeve remote from thetube end being seal welded to the periphery of the tube by a materialwhich will not produce electrolysis, a ferritic steel plate having abore therein, said sleeve and tube being disposed in said bore with thetube extending from one end ofthe bore and the end ofthe tube and sleevegenerally corresponding, with the face of the plate at the other end. ofthe bore and with said sleeve and tube being expanded. so the sleeve isin tight engagement with the bore, said sleeve and tube being Welded tosaid one face of the plate with the weld extending V.asggughout thecircumference of the. sleeve and tube.

2. In an organization of the type described a tube of a materialsubjected to stress corrosion cracking in the presence of certainchemicals, a sleeve disposed on the end of the tube and comprised of amaterial that is substantially unaffected by such chemicals and whichis. suffi.- ciently close to the metal of the tube in the electromotiveseries as to obviatey troublesome electrolysis, said sleeve and tubebeing integrally bonded together throughout their overlying surfaces, aplate also of a material substantially unaiected by such chemicals andhaving a. bore extend-ing therethrough, said sleeve and tube beingdisposed in said bore so the tube extends from one end thereof and beingexpanded so the sleeve is in tight engagement with the bore, the tubeand sleeve being welded to the plate at the other end of the boreandthroughout the circumference of the tube and sleeve and a seal Welddisposed -about the end of the sleeve corresponding with said one end ofthe bore to prevent electrolysis between the material bonding the sleeveto the tube and the tube.

3. In an organization of the typeV described a ferritic steel tube sheethaving a bore therein for receipt of a tube, an austemtie steel tube, ametallic sleeve disposed over the end of the tube and brazed thereto,with the brazing material being disposed throughout the entire overlyingarea of the sleeve and tube, said metallici sleeve being `of a materialthat is not subjected to stress corrosion` cracking in the presence ofchlorides, the end of the tube with the sleeve secured thereto beingdisposed inY said bore and expanded so the sleeve is in tight engagement with the wall of the bore with the remainder of the tube.extendinggfrom one end ofthe bore, the sleeve being seal welded tothetube sheet at the. other end. of the bore.

4. In a heat exchanger a plate into which the end of a tube isYconnected with theA tube being of a material subjected to stresscorrosion cracking from they presence of certain chemicals in water `incontact with the tube, andk the plate beting. of a material that issubstantially unaiected by the presence of such chemicals, the. tubebeing adapted to have a heating medium conveyed therethrough and watercontaining said certain chemicals disposed thereabout and evaporated bythe applicationI of heat from said medium, said end of the tube having asleeve disposed thereabout and brazed thereto throughout the overlyingsurface of the sleeve and tube with the sleeve being'` of a materialVsubstantially unaffected by said chemicals and which is sufficientlyvclose to the metal of the tube in the eleetromotive series as to obviatetroublesome electrolysis,. said sleeve being disposed witte in asuitable bore provided in the plate with the end of the tube adjacentone end of the bore, said sleeve and tube being welded to said plate atsaid one end of the bore with this weld extending across the surface ofthe sleeve and throughout the circumference of the tube and sleeve, anda seal weld disposed about the end of the sleeve remote from said oneend of the bore to prevent electrolysis between the brazing material andthe tube.

5. In an organization of the type described, a plate having a boreextending therethrough, said plate being of ferritic steel, anaustenitic steel tube having one end received within said bore with saidtube being adapted to have water containing chlorides disposedthereabout, said end of the tube having a sleeve disposed thereon whichextends at least the full length of the bore with said sleeve being of amaterial that is unaffected by chlorides, said sleeve and tube beingbrazed together throughout their overlying surfaces and said sleeve andtube being expanded so the sleeve is in tight engagement with the Wallof the bore while the tube remains spaced from said Wall, the end ofsaid sleeve adjacent said one end of the tube being circumferentiallywelded to the plate, and a =seal weld disposed about the end of thesleeve remote from said one end of the tube to prevent electrolysisbetween the brazing material and the tube.

References Cited in the file of this patent UNITED STATES PATENTS1,438,596 Harding Dec. 12, 1922 1,856,618 Brown May 3, 1932 2,209,974Jacobus Aug. 6, 1940 2,368,391 Young Jan. 30, 1945 2,678,224 KooistraMay 11, 1954 2,745,797 Long May 15, 1956

